1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a method and apparatus for locking the wavelength of a laser beam in a freespace laser communications system.
2. Background Information
Free-space laser communication systems transmit and receive information by means of a light beam that propagates through space or the atmosphere. When used for air-to-air or air-to-ground communications, such systems pose a number of challenging problems.
One such problem relates to picking targets out of bright backgrounds with wide field of view or long integration time optical acquisition and tracking systems. In such circumstances, background light rejection is essential. Typically, suitable detection systems in a receiving laser transceiver employ very narrow band optical filters, such as atomic line filters
A typical embodiment of a free-space laser communication system uses lasers as tracking beacons. That is, each transceiver in the system transmits a laser beam as a tracking beacon, and each remote transceiver acquires and tracks the tracking beacon of a transmitting transceiver in order to permit communications between the transceivers.
A common laser for use as a tracking beacon is a single mode diode laser. Although such diode lasers have very narrow spectral characteristics (typically less than about 20 MHZ), they can be tuned over a range of approximately 30 GHz. Further, the output of such lasers drifts in wavelength with various parameters including temperature and current. If the laser wavelength of a transmitting transceiver is set or drifts outside of the passband of the ALF of a receiving transceiver, no acquisition or tracking can occur. Therefore, it is necessary to employ some means of setting the current and temperature of a transmitting laser so as to generate an output wavelength that corresponds to the ALF passband of a receiving transceiver, even when the transmitting and receiving transceivers are separated by hundreds of kilometers.
Several approaches have been suggested to resolve this problem. One approach is to incorporate an output line filter in the transmitter of each transceiver. For example, a Faraday or Voigt ALF may be incorporated into the beacon laser assembly to filter the output of the laser beacon. One such embodiment is described in allowed U.S. patent application No. 08/221,527, filed Apr. 1, 1994 entitled "Point to Point Laser Communication Device" and assigned to the assignee of the present invention, the teachings of which are incorporated by reference. In such a system, the beacon laser light has the correct wavelength required to pass through the transmitter's output filter only if the drive parameters for the laser are set to the correct value. An external feedback mirror is used to direct light back to the laser cavity. The output coupler in such an implementation is typically about 80% transmissive, so about 20% of the signal is sent back through the ALF to the laser to lock the output wavelength at the necessary value. A monitoring optical detector, such as a photodiode, detects either a reflection off of the output optics or an intercepted portion of the post-filter laser beam. The optical detector is used in a feedback loop to set the laser drive parameters for maximum output at the selected wavelength.
There are two significant problems with this approach. First, the number of optical elements in the beam train make the ALF only about 60% transmissive at best. This is a significant reduction in output power, having a direct negative impact on communications range and data rate. Second, the use of optical feedback to lock the laser wavelength means that the system is very sensitive to vibration and shock, which change the distance between the laser cavity and the external feedback mirror. For air-to-air laser communication applications, the necessary mechanical stability is difficult to achieve.
A second approach is to lock the wavelength of the beacon laser using an external Fabry-Perot etalon. This approach is very similar to using the Faraday filter, except here the wavelength is locked to a resonant mode of the etalon. Unfortunately, there still needs to be some reference to the atomic absorption used in the Faraday receive filter, and the system suffers from the same mechanical stability problems encountered in the transmitter ALF approach described above.